9 Alarming Facts About Coral Reef Bleaching


Children form a reef fish on a beach in Hong Kong on April 23, 2015 to mark the 3rd annual Kids Ocean Day. The event is aimed at raising awareness of seas protection and the conservation of coral reefs worldwide. Image credit: Philippe Lopez/AFP/Getty Images

The longest-lasting recorded global bleaching event began in 2014 and continues to affect coral reefs worldwide. Few areas in the Southern Hemisphere escaped bleaching in the recently ended summer; surveys of the Great Barrier Reef suggest that more than 90 percent of it has been affected by bleaching. As heat arrives in the Northern Hemisphere, scientists expect to see bleaching in the Caribbean, Atlantic and Pacific, including Hawaii.

Scientists first recorded a mass coral bleaching, one which affects entire reef systems and not just a few individual corals, in 1979. Sixty recorded events occurred between 1979 and 1990. Global coral bleaching events are mass bleaching across all three tropical ocean basins—the Atlantic, Pacific, and Indian Oceans. The first global event took place from 1997 to 1998, with at least 15 percent of global reefs dying, and the second occurred in 2010. Number three, still happening today, looks on track to be the worst ever, affecting 38 percent of the world’s reefs.

Here’s what you need to know about this worldwide disaster.


A coral reef in the Red Sea near Obhor, north of Jeddah, Saudi Arabia. Image credit: Hassan Ammar/AFP/Getty Images

Thousands of species of coral exist, and hundreds of those are hard or reef-building corals. These individual coral animals, called polyps, look sort of like miniature sea anemones—a soft, tubular body topped by a ring of tentacles. Each builds a hard, protective external skeleton of calcium carbonate (CaCO3) around its base. Thousands together make up a reef, with each polyp connected to the next by a thin layer of tissue. Reefs grow when polyps periodically lift off their base and secrete a new one, adding layers of calcium. Sexual and asexual reproduction also increase the size of reefs by increasing the number of individual polyps.

Reef growth varies from 2 centimeters (0.8 inches) or less per year for massive corals up to 10 centimeters (about 4 inches) per year for branching corals. That means it can take thousands of years for a sizable reef to form and from hundreds of thousands to millions of years for barrier reefs and atolls.

Most reef-building corals grow best in water with temperatures between 23° and 29°C (73 to 84 F), high salinity (from 32 to 42 parts per thousand), and enough clarity to permit high light penetration. This generally restricts reefs to tropical or subtropical waters, (between 30° north and 30° south latitudes) and to the euphotic, or light-penetration, zone of the ocean, at most about 230 feet deep in the areas they call home.


A school of manini fish passes over a coral reef in Honolulu's Hanauma Bay. Image credit: Donald Miralle/Getty Images

Coral reefs function like oases in a desert, providing food and shelter for marine life. According to the Global Coral Bleaching consortium, coral reefs represent 0.1 percent of the world’s ocean floor, but help support approximately 25 percent of all marine species. For example, reefs shelter juvenile fish until they grow large enough to venture into the open ocean. That 25 percent of marine life represents the livelihoods of 500 million people and an economic asset worth $1 trillion.

NOAA estimates the commercial value of U.S. fisheries from coral reefs at more than $100 million annually. “Australia’s Coral Reefs: Under Threat from Climate Change,” a report from the Climate Council of Australia, notes that the Great Barrier Reef alone made a value-added economic contribution to the Australian economy of $5.7 billion in 2011–12, supporting 69,000 jobs. The Great Barrier Reef stretches more than 1400 miles and contains some 3000 individual reefs.

Coral reefs also protect shorelines from storms and contribute to local economies through tourism.


Coral reefs in the lagoon of the Toau atoll, about 250 miles from Tahiti in the Tuamotu Archipelago in the French Polynesia. Image credit: Gregory Boissy/AFP/Getty Images

Most coral polyps are transparent; the color of a reef comes from symbiotic algae, or zooxanthellae, living within the tissues of the polyps. Coral and zooxanthellae have a mutually beneficial relationship, the coral providing the algae with a home and the algae providing coral with 80 percent of its nutritional requirements through photosynthesis (hence the need for sunlight).

Corals experience stress when conditions move outside of normal ranges. Bleaching primarily results from temperature stress, when surrounding water temperatures are higher or lower than the coral organism’s optimum range. When stressed enough, corals expel their zooxanthellae, revealing the white calcium-carbonate skeleton and producing a “bleached” look. The animal is still alive at this point but only catching 20 percent of the food it needs. If the stress ends quickly enough, coral will accept algae back into its tissues and recover. If not, the coral will die. Its skeleton then becomes overgrown with other species of algae, which cover spaces where new coral polyps could land, preventing replenishment of live coral. This causes a shift from a coral-dominated to an algae-dominated reef, which has much less biodiversity.


A turtle on Australia's Great Barrier Reef. According to an August 2014 Australian government report, the outlook for the Earth's largest living structure is "poor," with climate change posing the most serious threat to the extensive coral reef ecosystem. The situation has only worsened since then. Image credit: William West/AFP/Getty Images

Rising ocean temperatures due to climate change bear responsibility for mass coral bleaching events. Global average temperatures have risen for more than 50 years, with the 10 warmest years on record globally occurring since 1998. Ninety-three percent of climate change heat is absorbed by the ocean. Corals haven’t adapted to these higher base temperatures and cannot cope with the prolonged temperature peaks, and the increased amounts of carbon dioxide has caused the ocean to become much more acidic, which slows the growth of coral even more.

Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration’s Coral Reef Watch, tells mental_floss, “The role of climate change in this event is huge. There is no way this event could be happening the way it is now if we weren’t seeing global warming.”

The surface temperature of the oceans increased by 1°C (33°F) during the past 35 years. A 2012 paper in Nature warned that a 2°C (35°F) rise in sea temperatures—an oft-stated goal set by the 2009 UN Climate Change Conference—will cost us at least one-third of the world’s coral reefs.

El Niño, a large-scale, ocean-atmosphere climate interaction in the tropical Pacific, creates unusually warm ocean temperatures in the Equatorial Pacific. It can significantly affect weather patterns and ocean conditions across large portions of the globe. Originally recognized by fishermen, the phenomenon was named El Niño as it tended to arrive around Christmas. Eakins points out that the 2015 El Nino weather pattern kicked in earlier than usual—March and April rather than June—and water temperatures were already so warm that it didn’t take long for bleaching conditions to occur.

Jodie Rummer, a senior research fellow at the ARC Centre of Excellence for Coral Reef Studies, who conducted aerial surveys of the Great Barrier Reef, blames the combination of El Niño, climate change, and an extended period of hot summer days with exceptionally low tides for bleaching of many corals there.


In May, the Climate Council of Australia reported that 93 percent of individual reefs in the Great Barrier Reef have suffered some degree of bleaching, with northernmost reefs most severely affected. This is by far the worst event in the reef’s history, the organization states. Analysis indicates, on average, 50 percent mortality of bleached corals north of Port Douglas, with a final death toll likely to exceed 90 percent on some reefs.

On U.S. reefs, record-breaking bleaching events have occurred in the Hawaiian Islands, American Samoa, Guam, the Commonwealth of the Northern Mariana Islands, and Florida. Severe bleaching has been seen on reefs across the Pacific, Caribbean and Indian Oceans.

At this point, an estimated 36 percent of coral reefs worldwide have been affected by major bleaching with nearly all reefs experiencing some thermal stress.


An aerial view of the Great Barrier Reef in August 2009. Image credit: Phil Walter/Getty Images

Scientists at universities and government agencies around the world monitor coral reefs on a regular basis, conducting regular surveys from airplanes and boats. The NOAA’S Coral Reef Watch program collects satellite data on environmental conditions, including sea surface temperatures, to quickly identify areas at risk for coral bleaching. The program also collects reports of visual observations of the health of reefs.

Scientists with the ARC Centre of Excellence for Coral Reef Studies at Australia’s James Cook University, who conducted aerial surveys of the Great Barrier Reef followed by in-water observations, recognized the unusual extent and severity of this event early this year.


When coral dies, fish and other animals that feed on or shelter in it die or move away. Larger creatures that feed on them also go away. This includes birds, and their loss can affect island plant ecosystems nourished by bird droppings.

Decline of populations of fish and other sea life affect human food supplies as well as the fisheries and tourism industries. This will create economic hardship for people with livelihoods connected to those industries. Loss of healthy reefs will increase the threat from storm surge—even as climate change increases the severity of storms—and coastal erosion.


University of Miami biologists Andrew Baker and Rivah Winter study staghorn coral in April 2016 as part of their research into how multiple climate stressors will impact coral reef in the future. Scientists around the world are studying how they can help coral survive the warming and acidification of the world's oceans. Image credit: Joe Raedle/Getty Images

Healthy corals have a greater ability to recover from bleaching, so it helps to reduce other stressors, such as overfishing and pollution, including agricultural and urban run-off. Many scientists have called for action in these areas and are supporting work to make coral reefs more resilient. The Nature Conservancy, for example, offers resilience workshops and training to reef managers.

Researchers also are using genetic analysis to identify coral species more tolerant to warming oceans and considering facilitating the spread of those species.

Addressing climate change remains critical, though. The Climate Council of Australia report states “…the future of coral reefs depends on how much and how fast we reduce greenhouse gas emissions now, and in the coming years and decades. Global emissions must be trending downwards by 2020 at the latest.”


A giant clam nestles among coral reefs in the Red Sea near Obhor, north of Jeddah, Saudi Arabia. Image credit: Hassan Ammar/AFP/Getty Images

“Reducing your carbon footprint is absolutely number one,” Eakin says. He also points out that many coral stressors result from local actions, including overfishing and poor land use that allows soil and nutrients to wash into the ocean.

If you directly interact with reefs by snorkeling or scuba diving, choose environmentally conscious operators who, for example, anchor away from reefs. Never touch coral (not even for that fantastic macro shot) and be careful your gear or fins don’t either. Avoid using sunscreen, as recent research shows it harms corals (wear protective clothing while diving and snorkeling instead).

Become a citizen scientist and contribute to global coral data collection. Columbia University and The World Surfing League developed an app called Bleach Patrol that allows anyone to report bleached or healthy reefs in real time, from anywhere in the world. “The Bleach Patrol dataset will help us understand the geographic extent and patterns of bleaching,” explains Eakin. “It tells us when and where corals are bleaching and when and where they are healthy.”